PSI - Issue 23

Jozef Majerík et al. / Procedia Structural Integrity 23 (2019) 541–546 Jozef Majerík et al. / Structural Integrity Procedia 00 (2019) 000 – 000

543

3

individual machining technologies used on the machined surface that leave so-called. particular characteristics and thereby consequently form a special state of the machined surface. Just by the force effect of the cutting tools, the machining process not only deforms the workpiece itself, which leaves the cutting zone in the form of chips, but also the thin surface layer just below the machined surface. Also, the cutting conditions used have a direct effect on the surface finish itself. Many experiments show that the cutting speed has the most significant effect. Therefore, the authors of this article chose the variable cutting parameter v c [m.min -1 ] as the variable, while feed rate f [mm.rev -1 ] and depth of cut a p [mm] were subsequently determined as constant cutting parameters. 2.1. Material characterization The OCHN3MFA steel (according to Russian GOST nomenclature) was selected and used as the experimental material in the performed experiments. It is a medium alloyed CrNiMoV steel with high strength and at the same time optimal plastic properties, which is used in industrial practice mainly for the production of major large caliber weapons, such as howitzers and tank guns. The equivalent of this steel in terms of its chemical composition is e.g. 35NiCrMoV12-5 steel, or American AISI 4340 steel. The chemical composition of 33NiCrMoV steel, which was measured by spectral analysis of experimental material, is shown in Tab. 1. The basic mechanical properties that have been determined on experimental material using standard methods according to the relevant standards are as follows. Tensile strength R m = 1500 MPa, limit of proportionality R E = 1079 MPa, Vickers hardness HV 500, toughness KCU = 23 J and elongation A 5 = 20 %. 2.2. Microstructural analysis In this process of experiments, all specimens which were used for AFM (Atomic Force Microscopy) metallographic observation were together prepared through standard metallographic sample preparation process. All the samples were wet grinded using of silicon carbide paper with grit from 80 down to 2000. Then were subsequently polished, and after this technique then finally etched with using Nital 3% as the etchant . Atomic force microscopy AFM represents an uncommonly specific used method for investigating micro and nano-structure of researched metallic materials. The fundamental principle of this used microscope is to scan the specimen surface with a small and specific geometry tip, which has a tip-radius of only a very few nanometers. In the material investigation, the most important testing modes are those that can measure some parameters as are for example Young module, indentation, hardness, or dissipation. The Oxford Instruments MFP-3D Infinity AFM type of used microscope have been performed for measurements realization and allows for exploring material properties with using specific AM-FM Viscoelastic Mapping Mode. Applied AM-FM mode can register for us some nano mechanical properties of investigated materials across a wide range of Young modules directly from biomaterials to non-ferrous and ferrous metals or industrial ceramics. In the process of experiments, used AM-FM mode summarized obtained results by measuring at two cantilever resonances simultaneously. The first resonance is applied for tapping mode imaging, which is also known as amplitude modulation (AM), while the second resonance mode usually works in frequency modulation (FM). In resonances, cantilever frequency and phase respond sensitively to changes in sample properties. Fundamentally, greater frequency means higher value of stiffness or modulus. Indentation is yet another parameter describing mechanical properties of surface. Indentation represents deformation of surface when tip is tapping on it. 2.3. Nano-hardness analysis of surface layers after machining All measurement of nano-hardness was directly realized on Hysitron TI-950 Triboindenter investigating device 2. Experimental work Table 1. Chemical composition of examined 33NiCrMoV steel preformed on the spectral analyzer Spectrolab Jr CCD. Component C Mn Si Cr Ni Mo V P S Weight % 0.403 0.3 0.32 1.19 3.275 0.523 0.1363 0.01 0.01